In the field of magnetic recordings, especially in the field of magnetic disks, performance is enhanced by making the medium smaller and thinner and increasing its capacity, but this has to accommodate high precision of the magnetic recording medium. Recently, there has been active discussion on glass substrates with high rigidity and high hardness that can be easily smoothed and thus are very useful to increase density and reliability.
Hitherto, in the course of manufacture, after cutting the glass substrates for magnetic disks into a predetermined size, the glass substrates were polished one by one in a polishing step to obtain a smooth surface. However, there were the problems, that the polishing step has to be performed with high precision and the number of processing steps is large.
On the other hand, a press-molding method for the field of optical glass element production that enables high product quality and high productivity is widely discussed and its practical use is already underway.
As a press-molding die, a special die that does not deteriorate when glass is repeatedly molded is necessary and has been variously discussed.
Hard metal (tungsten carbide), cermet, zirconia, silicon carbide and other ceramics are used as a base material for press-molding dies, and a die has been developed that is coated with a protective layer with good mold releasing properties, oxidation resistance and chemical resistance, to protect the base material and prevent adhesion of the glass to the die at the time of mold release.
For example, Publication of Unexamined Patent Application (Tokkai) No. Hei 2-137914, discloses a die having a noble metal alloy film applied to a hard metal surface, and a fine pattern formed on the alloy surface. However, there were problems in that the hard metal (tungsten carbide) or the cermet used as a base material has poor processability, that it is difficult to obtain sufficient smoothness (on the nm order) to mold substrates for magnetic disks and that defects occurred frequently in the surface at the time of processing.
Moreover, the fine processing for the noble metal alloy film is usually very troublesome and the desired pattern is difficult to obtain, so that the quality of the obtained magnetic disk substrates was often inferior.
In optical elements such as optical lenses and prisms, small irregularities in the surface of the die can lead to an increase of scattered light, so that lenses with a smoother surface are desirable, yet conventional molds reached their limit.
Various techniques have been presented that use, as a molding base material, a glass material in which sufficient smoothness for use as a substrate for magnetic disks can be easily attained. Tokkai No. Hei 1-148714 discloses glass as the base material for press-molding, and fabrication of a protective layer of a ceramic or a noble metal material. Tokkai No. Sho 64-33022 suggests a die for molding an optical element using a fluoride such as MgF.sub.2 or an oxide such as SiO.sub.2 as a mold release film. Tokkai No. Hei 2-51434 discloses a die for molding optical disks with glass as the base material for the press-mold, having a film with a fine pattern formed by fine processing on the surface, wherein a carbon film as a protective layer and an intermediate film of silicon carbide and silicon nitride are formed.
However, the above examples do not have enough durability to withstand the repeated use for press molding under severe heat and pressure, so that the surface roughness of the press surface increases with long-term repeated use. Moreover, exfoliation of the microscopic protective layer may occur and the product quality may deteriorate due to changes in the fine pattern, so that these techniques cannot adequately answer the recent demand to achieve higher durability and longer life-expectancy.
Conventionally, a grinding method, performed by rotating a disk-shaped diamond grinding stone, was used to process the mold, but so far, a disk-shaped diamond grinding stone with a diameter of less than 2 mm could not be produced, and a concave die of a smaller diameter could not be processed, which posed a limit to the production of optical glass elements by the press molding method.